454 LESLIE B. AREY AND W. J. CROZIER 
poison glands projecting, the body is admirably shaped for rolling 
over to one side or the other, even before twisting is begun, the 
form thus tending to facilitate righting. We have never seen an 
Onchidium in nature disturbed to an extent sufficient to bring 
this behavior into plaj^ It is unlikely that they are ever dis- 
lodged by wave action, even though the force of attachment 
through the foot is not great, since they do not creep upon the 
exposed rocks under water; nor do they come out of their 'nests' 
at low tide when there is a stormy wind; nor do they on the 
whole inhabit places where wave action is severe, but rather the 
reverse. 
In nature, Onchidium must spend a good part of each day 
under water, sometimes several days continuously. Certain of 
the nests may entrap a small amount of air as the tide rises, but 
this cannot be the case with most of the nests. In aquaria, these 
animals will live for at least two weeks under sea-water, without 
visible impairment. In several instances groups of them were 
forced to do so by being placed upon a stone suspended by a 
string under water. A group isolated in this way stays during 
daylight hours, and also for the greater part of the night, on the 
under (shaded) side of the stone. 
When beneath the water surface an Onchidium tends to creep 
more slowly than when in air, and remains for long periods 'at 
rest,' with girdle depressed, dorsum arched, tentacles retracted. 
While creeping under water the tentacles are never so far pro- 
truded as they characteristically are in air. As already noted, 
the attachment of the foot to the substratum is not very firm in 
Onchidium; it is, however, quite sufficient to enable the creature 
to creep on the under surfaces of stones in air as well as under 
water. 
Onchidium emerges from its nest at low tide even though rain 
may be falling heavily, provided no wind is blowing strongly 
on the bit of shore concerned. It is therefore interesting to 
determine the toxicity of rain-water for Onchidium, especially 
since the porous rocks containing the nests of these mollusks 
must frequently permit the seepage of rain-w^ater into the Onchid- 
ium shelters. Experiment showed that an Onchidium placed 
